Intraluminal prostheses used to maintain, open, or dilate blood vessels are commonly known as stents. Stent constructions generally include lattice type cylindrical frames that define a plurality of openings. Common frameworks for stents include, for example, individual rings linked along the length of the stent by a linking member, a continuous helically wrapped member (that may include one or more linking members), a braid or a mesh formed into a tubular structure, and a series of interconnected struts. Stents may be formed by arranging one or more members in a pattern along a longitudinal axis to define essentially a cylinder and connecting the one or more members or otherwise affixing them in position (e.g., interconnecting with a filament). Stents may also be formed by cutting openings into a tube of material (e.g., shape memory).
Stents may have self-expanding and/or balloon expandable properties. Self-expanding stents are delivered to a blood vessel in a collapsed condition and expand in vivo following the removal of a constraining force and/or in the presence of an elevated temperature (due to material properties thereof), whereas balloon expandable stents are generally crimped onto a balloon catheter for delivery and require the outwardly directed force of a balloon for expansion. Stents can be made of various metals and polymers and can include a combination of self-expanding and balloon expandable properties.
Synthetic vascular grafts are routinely used to restore the blood flow in patients suffering from vascular diseases. For example, prosthetic grafts made from expanded polytetrafluoroethylene (ePTFE) are commonly used and have shown favorable patency rates, meaning that depending on a given time period, the graft maintains an open lumen for the flow of blood therethrough. Grafts formed of ePTFE include a microstructure characterized by spaced apart nodes connected by fibrils, the distance between the nodes defined as internodal distance (IND), and are generally extruded either as a tube or as a sheet or film that is fashioned into a tube. Grafts can also be created from fibers woven or knitted into a generally tubular shape.
It is known in the art to use stents in combination with vascular grafts to form stent-grafts. Because stent-grafts are often intraluminally deployed in vessels of varying sizes and tortuosity, flexibility can be an important consideration. Flexibility can be imparted to a stent-graft in a variety of ways, including, for example, connection of the stent to the one or more graft layers, configuration of the stent and/or graft layer(s), spacing of the stent struts, rings, or members along the length of the graft(s), etc. For example, U.S. Pat. No. 6,398,803 and U.S. Pat. No. 6,770,087 to Layne et al., which are incorporated by reference in their entirety into this application, describe a graft layer with openings to enhance flexibility. U.S. Pat. No. 6,558,414 to Layne, which is incorporated by reference in its entirety into this application, describes another type of graft layer to enhance flexibility.
Another important consideration in the design of a stent-graft is the ability of the stent to withstand stress and fatigue, caused, for example, by plastic deformations occurring at strut junctions when the stent is subjected to circumferential forces. Stent strength can be enhanced through material choice, stent configuration, arrangement and configuration of graft layers, connecting members between stent members, etc. Yet another consideration in the design of a stent-graft is its columnar strength in the compressed configuration to enable the stent-graft to be pushed out of a delivery sheath, for example, without buckling or deforming. This consideration, however, must be balanced with flexibility considerations, such as those discussed above.
The following references relate to stents and stent-grafts: U.S. Pat. No. 5,591,197 to Orth et al.; U.S. Pat. No. 6,695,833 to Frantzen; U.S. Patent Application No. 2003/0229390 to Ashton et al.; and U.S. Patent Application No. 2004/0230288 to Rosenthal, each of which is incorporated by reference in its entirety into this application.
Applicants have recognized that it would be desirable to provide a stent-graft that is relatively stiff in a collapsed configuration and more flexible in a deployed expanded configuration, embodiments of which are described herein along with methods of making same.